1. Field of the Invention
The present invention relates to a light emitting device, more particularly to a light emitting device employing non-stoichiometric tetragonal Alkaline Earth Silicate phosphors, which can be used as temperature stable luminescent material for ultraviolet or visible light excitation.
2. Discussion of the Background
A light emitting device (LED) has been widely used for indicators, display boards and display devices since the light emitting device can implement colors. White light can also be implemented, so that the light emitting device has been used for general illumination. Since such a light emitting device has high efficiency and long life span and is ecological, the fields in which the light emitting device is used have been continuously extended.
In the field of LED, various methods for implementing colors, for example white light, have been suggested, and the technique to dispose phosphors around a light emitting diode and combine light emitted from the light emitting diode and light emitted from the phosphors is generally used to implement white light.
In the meantime, stoichiometric silicates such as Orthosilicates, Disilicates and Chlorosilicates are well known as converter materials for short or long wave excitation like ultraviolet as well as daylight radiation. (G. Roth; et al. “Advanced Silicate Phosphors for improved white LED” (Phosphor Global summit Seoul/Korea, Mar. 5-7, 2007))
Especially, silicate phosphors excited by blue light from an LED and emitting converted light are used in light emitting devices for implementing white light or color for demand for several applications. In the last years, the use of silicates has been increasing for LED applications.
The LEDs and especially the High Power LEDs produce a lot of heat during operation. Additionally, LEDs have to withstand high ambient temperature above 80° C. Phosphors themselves have a system depending on temperature-behavior. The brightness of most phosphors is decreasing with increasing temperatures.
This so-called temperature quenching depends on the interactions between activator and host lattice and is influenced by the composition of the matrix, structure, lattice effects, concentration as well as the kind of activator. In particular, the strength of the bonding within the crystal matrix is influencing the extension of the lattice parameters and from this the emission properties of the activator ions.
Furthermore, by increasing the temperature the oscillation of the ions within the lattice becomes higher. Because of this, the probability of an interaction with the activator ions becomes higher, resulting in an increasing loss of exciting energy in form of heat. This so-called Photon-Photon Coupling strongly depends on the structure and the surrounding of the activator ions. Greater rigidity in the crystal lattice results in less interaction between ions and activator.
The brightness of Orthosilicates, Disilicates as well as Chlorosilicates activated by divalent Europium decreases strongly with higher temperatures up to 150° C. because the lattice is not so rigid and the strength of the bonding is not so high.
This effect leads e.g. to a changing of the color of the LED during operation. This is a serious disadvantage of the use of common Silicates known until now for LED applications. Furthermore, the sensitivity against water is comparably high caused by the weak lattice and a highly heteropolar bonding between the Silicate ion and the Alkaline Earth ions.
Silicate phosphors have been developed in the recent years as luminescent materials for white LEDs. (WO 02/054503, WO 02/054502, WO 2004/085570)
Orthosilicates as luminescent material with an excitability from short ultraviolet radiation up to visible light can be used as phosphors for fluorescent lamps. (Barry, T. L., “Fluorescence of Eu2+-activated phases in binary Alkaline Earth Orthosilicate systems,” J. Electrochem. Soc., 115, 1181 (1968))
Co-doped Tristrontium-silicates are disclosed as yellow-orange luminescent material (H. G. Kang, J. K. Park, J. M. Kim, S. C. Choi; Solid State Phenomena, Vol 124-126 (2007) 511-514), Divalent europium as activator for silicates (S. D. Jee, J. K. Park, S. H. Lee; “Photoluminescent properties of Eu2+ activated Sr3SiO5 Phosphors,” J. Mater Sci. 41 (2006) 3139-3141 and Barry, T. L.; “Equilibria and Eu2+ luminescence of subsolidus phases bounded by Ba3MgSi2O8, Sr3MgSi2O8 and Ca3MgSi2O8,” J. Electrochem. Soc., 115, 733, 1968), and fluorescence for excitation by UV and blue radiation is disclosed in several Silicate systems as Orthosilicates and Disilicates. (G. Blasse, W. L. Wanmaker, J. W. ter Vrugt and A. Bril; “Fluorescence of Europium2+-activated silicates,” Philips Res. Repts 23, 189-200, 1968).
All these phosphors have the disadvantage that they have strong temperature quenching and a strong shift of the emission band with the temperature. The emission intensity can be dropped down to 50% at 150° C.